wsrep_causal_reads error on backup of MySQL8.0.19 with Galera 4

200602 19:07:36 [01]        …done200602 19:07:36 >> log scanned up to (58095061567)200602 19:07:36 Executing FLUSH NO_WRITE_TO_BINLOG TABLES…200602 19:07:36 Executing FLUSH TABLES WITH READ LOCK…Error: failed to execute query ‘SET SESSION wsrep_causal_reads=0’: 1193 (HY000) Unknown system variable 'wsrep_causal_reads’
xtrabackup version 8.0.12
mysql  Ver 8.0.19 for Linux on x86_64 (MySQL Community Server - GPL) wsrep_provider_version       | 4.5(rbf205c6)

Please file the bug at https://jira.percona.com/projects/PXB/summary 
In General Percona Xtrabackup was not tested with   MySQL with Galera Patches and we do not have plan to do so.  If you are looking for MySQL8 with Galera replication functionality we recommend Percona XtraDB Cluster 
https://www.percona.com/software/mysql-database/percona-xtradb-cluster